feat(cli): populate Git SHA, target triple, and build date at compile time via build.rs

Add rust/crates/rusty-claude-cli/build.rs that:
- Captures git rev-parse --short HEAD at build time → GIT_SHA env
- Reads Cargo's TARGET env var → TARGET env
- Derives BUILD_DATE from SOURCE_DATE_EPOCH / BUILD_DATE env or
  the current date via `date +%Y-%m-%d` fallback
- Registers rerun-if-changed on .git/HEAD and .git/refs so the SHA
  stays fresh across commits

Update main.rs DEFAULT_DATE to pick up BUILD_DATE from option_env!()
instead of the hardcoded 2026-03-31 static string.

Before: `claw --version` always showed Git SHA: unknown, Target: unknown,
Build date: 2026-03-31 in local builds.
After:  e.g. Git SHA: 7f53d82, Target: aarch64-apple-darwin, Build date: 2026-04-08

Generated by droid (Kimi K2.5 Turbo) via acpx (wrote build.rs),
cleaned up by Jobdori (added BUILD_DATE step, updated main.rs const).

Co-Authored-By: Droid <noreply@factory.ai>

rust/crates/rusty-claude-cli/src/main.rs

@@ -67,7 +67,12 @@ fn max_tokens_for_model(model: &str) -> u32 {
         64_000
     }
 }
-const DEFAULT_DATE: &str = "2026-03-31";
+// Build-time constants injected by build.rs (fall back to static values when
+// build.rs hasn't run, e.g. in doc-test or unusual toolchain environments).
+const DEFAULT_DATE: &str = match option_env!("BUILD_DATE") {
+    Some(d) => d,
+    None => "unknown",
+};
 const DEFAULT_OAUTH_CALLBACK_PORT: u16 = 4545;
 const VERSION: &str = env!("CARGO_PKG_VERSION");
 const BUILD_TARGET: Option<&str> = option_env!("TARGET");